簡介：在獲取巖心進行實驗室分析的復雜油藏中，通常含有疏松砂巖。對巖心進行恰當的分析帶來的效益通常是其成本的10倍。因此，花費這些費用是有必要的。
Unconsolidated sands are among the most difficult type of reservoirs in which to obtain core and to analyze in the laboratory. With the value of a proper core analysis typically exceeding 10 times the cost, it pays to get it right.
It is critical to know how much strain a sample undergoes during loading to net-effective stress. Diameter and length are monitored during testing with a radial strain gauge attached to a viton sleeve.
Excessive grain movement in unconsolidated sands beyond a nominal elastic limit prevents adequate restoration of pore structure and grain fabric to yield sufficiently accurate core analysis results. The mechanical stability of the core is the key to understanding whether a particular measurement is valid.
Experience shows that if laboratory-observed bulk volume strain is on the order of 5% or less, core data appear to correlate well with log data and other geologic and field data. Some measurements, of course, are more sensitive to damage. Intuitively, the most sensitive measurements to damage would be those that require preservation of good grain-to-grain contacts. These include velocity, compressibility, and rock mechanics. Measurements with slightly less sensitivity would include absolute permeability, electrical properties, capillary pressure, relative permeability, and porosity.
An argument could be made that with relative permeability, slight damage could be tolerated in the case where wettability dominates curve shape. This might be supported by observations where the amount of mobile oil is controlled by the approximate level of absolute permeability.
To achieve small values of bulk volume strain, extreme measures must be taken during coring, handling, transport, and with laboratory practices. These methods include sharing of best practices developed from other unconsolidated projects and include coring equipment, coring parameters, mud types, controlled trip-out, preservation and transportation with dry ice, special laboratory equipment, and protocols.
Unconsolidated sands recovery
There is often an assumption that a credible analysis of an unconsolidated core cannot be achieved. But if done properly, fine-scale turbidite flow structures can be well preserved and shown to present realistic texture in thin-section photomicrographs and scanning electron microscope (SEM) images. Shell accomplishes this with the use of dry ice freezing from lay-down to transportation and all phases of handling and laboratory processes.
Proficient coring and core recovery practices at the rig are essential to obtaining meaningful data for unconsolidated sands. Following the recommended trip time preserves the integrity of the core. Recommended trip times ? based on rate of pressure equalization across low-permeability mud cake ? must be calculated and followed. The nearer the core is to the surface, the slower the pull time. The greatest gas expansion is experienced in the last couple of stands.
Shell’s CT scans reveal that if a core is pulled too quickly, expanding gas from a high-permeability core center pushes against low-permeability mud-invaded skin. Pressure builds, creating radial and circumferential fractures that allow gas to escape (Figure 1). If damage is severe, material slump can occur. In this case, the recovered core exhibits disturbed annular material that suggests a distressed interior (Figure 2).
Once the core reaches the surface, the next important step is to lay down the core, maintaining the original pore structure and grain fabric. Coring contractors should use a shuttle device that supports the core rigidly during lay-down. The core needs to be packed in dry ice and frozen as soon as possible once it reaches the surface. Failure to adhere to trip tables and shortening freezing time contribute to poor core condition and uncertainty in analytical results.
Dry ice and freezing
In a hydrocarbon-bearing zone there is enough blow-down gas present to accommodate expansion of liquids. It is precisely the freezing of liquids that locks grains together and prevents inelastic deformation of the core. In other words, the grains do not move apart further than can be restored during restressing of the core.
Even for fully water-saturated shales where freezing damage is more likely, Shell has found that more mechanical damage has been caused by not freezing the core than by freezing it. Even after freezing, credible velocity and rock mechanics measurements for seismic modeling and wellbore stability design programs can be made.
Shell has never achieved a credible analysis on core preserved with anything other than dry ice. CT scans reveal that core preserved with gypsum exhibits massive damage throughout the entire core, failing to yield any usable plugs for analysis. CT scans of core preserved with dry ice, on the other hand, reveal minimal damage, yielding a high plugging success rate.
Experience handling cores is also critical. Analysis contractors should have not only sufficient experience, but also equipment for storing core material and plugs on dry ice as well as cryogenic freezers for preservation. Most oils will not freeze at normal electrical freezer temperatures of 4ºF (-20ºC). This can result in a core that remains soft and pliable, which creates potential for excessive grain movement and damage.
Failure to cut, recover, or maintain core properly can yield plugs that are unusable. It is critical that best practices are followed during plug preparation.
Using a high-quality milling machine minimizes mechanical vibration during drilling and helps prevent fractures and damage. The drill bit should be cooled and lubricated with liquid nitrogen to prevent heat buildup within the plug. Using dry ice during drilling keeps the core sticks frozen. Core sticks should never be allowed to thaw while plugging. Failure to maintain proper temperature risks the integrity of the entire core. Plugs that have been allowed to thaw are not usable.
Laboratory measurements
All special core analysis measurements, such as stress brine porosity and permeability, resistivity index, and mercury capillary pressure, should be made at reservoir stress conditions using equipment designed for the difficulties associated with handling unconsolidated sands. Plugs must be sleeved, mounted frozen, and not allowed to thaw until under stress.
Extraction should be accomplished miscibly flowing chloroform methanol/ methanol until all hydrocarbon and salts are removed. Once cleaned, porosity, permeability, and formation resisting factor should be measured simultaneously, avoiding extra stress cycles. For shaly sands, conductivity of rock as a function of water in pore space and multipoint stress also needs to be measured without additional loading. Automation of pumps and data acquisition improves efficiency and accuracy.
Stress cell features
Stress cells should be quick-loading in design so that samples do not thaw until under stress. The lab system should be designed to load sample, seal cell, and bring up to confining pressure in less than one minute before a sample thaws.
Cells should be temperature-controlled to help control confining stress variations. Stress cells should never be left in ambient conditions ? even minor temperature variations are sufficient to cause fluctuations in confining stress that will alter sample properties.
Bulk volume strain measurement
Teflon jacketing with sintered metal frits (refractory screens) is preferable to using the common foil/screen industry-standard arrangement that often suffers from wrinkling of the foil and consequent bypass. Heat-shrunk Teflon also adds rigidity, enhancing the ability for multiple measurements.
Knowing the bulk volume is critical. Damaged samples excessively compact with strains exceeding 5%. It is important that the diameter and length are constantly monitored during testing. This can be accomplished in a system with a radial strain gage and axial linear variable differential transducers (Figure 3). The stress cell should also be isostatically balanced using a load piston designed to compensate for the end-cap area. Applying unequal stresses in standard cell designs squeezes (or pinches) the sample like a tube of toothpaste.
The value of a proper core analysis typically exceeds 10 times the cost. From a risk discount point of view, that means every 1% reduction of uncertainty on a billion-dollar investment yields a value in excess of US $10 million dollars.
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